This resubmission focuses on signaling by Notch, a transmembrane receptor that influences cell fate choices in lower eukaryotes and which can alter proper development of particular cell lineages in higher eukaryotes, including T lymphocytes. The proposed experiments stem from the investigator's observation that Notch signaling can inhibit E47, a transcription factor that plays a critical role in early B cell development. He proposes, by extension, that Notch may also influence B cell development. The proposal takes a broad view and attempts to understand Notch from the phenotypic consequences of its activation to its molecular mechanisms of action. A combination of cell culture systems and transgenic mice will be used to investigate the effects of Notch signaling on gene expression in and development of B cells. Transgenic mice harboring a Notch responsive reporter will be generated in order to identify particular cells in the developing embryo and the adult that undergo Notch signaling. These latter experiments may provide insights into the mechanisms that govern cell fate decisions in a variety of cell developmental contexts. Finally, experiments are proposed to elucidate the molecular details of Notch's activation of the transcription factor CBF1. Recent results have shown that Notch inhibition of E47 does not involve signaling to CBF1 and, instead, may involve a novel pathway involving a second Notch-interacting protein called Deltex. Experiments are proposed to characterize signaling through Deltex.